Digital imaging systems employ focal plane arrays to sense image information. One important class of focal plane arrays is infrared sensing arrays. These arrays are useful for image detection and motion sensing. Infrared arrays detect infrared radiation that is given off by virtually all objects, including the detector array's components, in proportion to the objects temperature.
Different semiconductor materials are inherently sensitive to different portions of the electromagnetic spectrum as a result of their electronic energy band structure. Indium antimonide (InSb) and Mercury Cadmium Telluride (HgCdTe or MCT) are well known materials which are suitable for the detection of infrared radiation. While these materials are suited for infrared detection, they are not suitable for the formation of integrated circuits or other electronics to process the image information which is collected by the FPA formed on these materials. Consequently, it is the standard practice in the infrared sensing art to connect an infrared sensor from one of these materials to silicon based integrated circuits for processing of the image information produced from the infrared sensor. Thus the sensors are fabricated separately from the readout circuits and then mounted to a common substrate or circuit board. Alternatively, the sensors are fabricated on a piece of sensor material that has be mounted to the readout integrated circuit substrate.
One approach to fabricating FPAs for infrared digital imaging systems has been to create an array of p-n junction or heterojunction diodes that convert photons of a range of infrared frequencies into electronic signals to perform as optical detectors. Each diode in the array then defines a pixel within the photodetector array. These diodes are typically reversed biased and generate a current flow in proportion to the number of photons that strike the diode having a frequency which exceeds the band gap energy of the infrared material used to fabricate the diodes. The current flow for each diode can be monitored and processed to provide a digital image corresponding to the infrared energy incident to the diode array.
The diodes in the array are each formed as a junction of n-type and p-type semiconductor materials which define receptor regions for each photodetector. The materials used to fabricate the infrared detectors or photo diodes are typically semiconductors having elements from Group II and Group VI of the periodic table, such as mercury cadmium telluride (MCT). Using these materials, detectors have been used which operate in the lower infrared frequency band down to the limits of the available long wave length atmospheric window, i.e., at wavelengths of 8-12 microns. The detection of such long wavelength radiation, if it is to be done at only moderate cryogenic temperatures, e.g. at liquid nitrogen rather than liquid helium temperatures, is preferably done using a very narrow band gap semiconductor such as MCT. Compositions of MCT having a selectable band gap energy may be specified by varying the proportions of mercury and cadmium in the composition Hg.sub.1-x Cd.sub.x Te, hereinafter referred to generally as MCT.
In the formation of these photodetectors it is important to include a protective layer such as cadmium telluride (CdTe) on the MCT wafer to act as a passivation layer, antireflective coating and/or an insulator for conductive interconnect lines. Passivation of MCT during detector fabrication has been found to reduce dark currents arising from surface states. Dark currents are spurious currents which flow despite the complete lack of infrared light at the frequencies the detector is designed to detect. Dark currents thus are error currents or leakage currents across the junction of the diodes. They are caused by imperfections in the bulk or surface of the MCT. Dark currents which occur at the surface of the MCT are particularly troublesome. Dangling bonds at surfaces can contribute to surface imperfections which alter the electrical characteristics of the detectors, such as, the photocarrier lifetimes and surface recombination velocity. Other imperfections include extrinsic and intrinsic impurities, or dislocations of the MCT.
The surface imperfections can be reduced by application and interdiffusion of a passivation layer. Cadmium telluride (CdTe) has generally been used as the passivating material in the prior art. The CdTe is deposited on the MCT and heated to about 300.degree. C. for several hours. The mercury then diffuses into the CdTe and the cadmium diffuses into the MCT to provide a graded rather than abrupt interface. Interdiffusion of the CdTe layer and the MCT layer eliminates the dangling bonds of the MCT layer and diffuses any remaining impurities away from the MCT surface.
Embodiments of the present invention are directed to the topside illuminated, or Vertically Integrated Photodiode (VIP) approach for fabricating FPAs. In this approach, a slice of group II and/or group VI elements such as MCT is epoxy mounted to a Read Out IC (ROIC). A ROIC is typically a silicon chip which has contact pads for each pixel of the detector array prefabricated on the silicon, in addition to circuitry for monitoring and processing the output of the photodiode detector array. After the diodes are formed on the MCT slice, the diodes are connected to the ROIC by etching holes through the MCT and connecting each diode to a corresponding contact pad on the ROIC with metal leads. This process in described in U.S. Pat. No. 4,720,738 issued to Arturo Simmons and incorporated herein by reference.